Academic journal article The American Midland Naturalist

Habitat Use by Loggerhead Shrikes (Lanius Ludovicianus) at Midewin National Tallgrass Prairie, Illinois: An Application of Brooks and Temple's Habitat Suitability Index

Academic journal article The American Midland Naturalist

Habitat Use by Loggerhead Shrikes (Lanius Ludovicianus) at Midewin National Tallgrass Prairie, Illinois: An Application of Brooks and Temple's Habitat Suitability Index

Article excerpt

ABSTRACT.-Declines in loggerhead shrike populations have been attributed to pesticide use and habitat loss on the breeding grounds and factors outside the breeding range. To determine the role of breeding habitat limitation, Brooks and Temple (1990) designed a habitat suitability index for shrikes based on data from Minnesota. This paper describes an application of their model to a site in Illinois. Like Brooks and Temple, I found that breeding habitat does not appear to limit shrike populations and shrikes seem to be making settlement choices based on discernable habitat criteria. I suggest changes to the model for adaptation to Illinois shrike populations, including an adjustment of the cutoff for "suitable" habitat, an adjustment of the conversions of variables leading to the calculation of the index (V4 to SI4), the use of GIS to measure variables (usable foraging habitat) and the addition of variables (length offence) used in the model.

INTRODUCTION

The loggerhead shrike (Lanius ludovidanus) is a passerine that prefers shorter grasses, including pasture with scattered trees and shrubs (Kridelbaugh, 1982; Smith and Krusc, 1992). Declines in loggerhead shrike populations in the Midwestern United States have been attributed to changes in agricultural practices including pesticide use and habitat destruction (Graber et al., 1973; Yosef, 1996). Brooks and Temple (1990) suggested that, for some migratory populations in Minnesota, this decline is due to events occurring outside the breeding range. This suggestion was based on their finding that more suitable habitat was available for shrikes than was used. However, no single factor has been shown to be responsible for their decline.

Managers who create or protect areas intended for breeding shrikes require knowledge of specific breeding habitat requirements. Brooks and Temple (1990) developed an index to assess the suitability of habitat for loggerhead shrikes breeding in southern Minnesota. This model has not been applied to shrike populations in Illinois but, if applicable, it could prove useful in guiding management decisions. In this study I applied the habitat suitability index developed by Brooks and Temple (1990) to a shrike population in northern Illinois. This Illinois population represents a significant portion of this species within the Prairie Parklands macrosite and within northeastern Illinois (USDA-Forest Service, 2001). My objectives were: (1) to determine if suitable habitat, as identified by Brooks and Temple (1990), is limiting in northern Illinois; and (2) if necessary, modify the suitability index to better fit the habitat structure in Illinois.

STUDY AREA AND METHODS

Midewin National Tallgrass Prairie (MNTP) consists of approximately 6475 ha in Will County, Illinois, about 70 km southwest of Chicago. Formerly owned and operated by the United States Army as the Joliet Army Ammunition Plant, this land was transferred to the United States Department of Agriculture, Forest Service (USFS) in 1997. MNTP is now managed jointly by the USFS and the Illinois Department of Natural Resources (IDNR) with a goal of restoring native prairie communities. The loggerhead shrikes at MNTP belong to a migratory subspecies, Lanius ludovicianus migrans. These birds arrive at MNTP in spring (April-May), breed and leave during fall (August-October).

From 1988 through 1991, loggerhead shrikes were monitored at MNTP on an ad hoc basis by IDNR personnel. Nest locations were recorded when located by field ecologists but no data were collected regarding nest success. Beginning in 1994, data were collected on nest location and height, nest tree species and height, number of eggs/nestlings inside the nest, approximate age of the nestlings, nest fate (fledged, depredated or abandoned) and the presence of adults. During this phase of monitoring, nests were typically checked once every 7-10 d.

From March to August 2000 and again from May to july 2001, nests were located and monitored every 3-4 d. …

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